The Apollo moon landing missions used a Lunar Orbit Rendezvous (LRO) approach where the Command Service Module would be in a low lunar orbit (LLO) while the Lunar Module would land on the moon. This approach saved a significant amount of delta-v when compared to the direct ascent approach, because the heavy CSM did not need to be landed on the moon.

Did NASA consider extending the LOR approach to have the CSM be in a highly elliptic orbit around the moon, for example 50,000 km by 110 km, which should save delta-v compared to the LLO? If it was considered, why was the LLO approach used instead of the highly elliptic lunar orbit approach?

  • $\begingroup$ Would you have to budget fuel anyway to get down to LLO if the LM did a descent abort, had ascent issues, or otherwise needed to be retrieved in an unplanned manner? $\endgroup$ – Organic Marble Feb 1 '17 at 20:56
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    $\begingroup$ Later missions with heavier surface cargos did use an elliptical orbit, but not as you describe - they used the CSM rather than the LM for the Descent Orbit Insertion burn, effectively transferring some of the delta-V burden from the LM to the CSM. This might seem inefficient from a high-level perspective but in reality the CSM had some extra performance margin while the LM did not, so this was the way to utilize it. $\endgroup$ – pericynthion Feb 1 '17 at 21:25
  • $\begingroup$ @pericynthion You should expand that into an answer. $\endgroup$ – Russell Borogove Feb 1 '17 at 21:29
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    $\begingroup$ I'll second that -- make that another answer. It is interesting that they did the exact opposite of what you'd expect. They executed more $\Delta V$ on the more massive full stack and less on the LM, which is less efficient overall. But that doesn't matter if you can't put more propellant in the LM tanks. $\endgroup$ – Mark Adler Feb 2 '17 at 5:35
  • $\begingroup$ You don't save delta-v by taking one craft instead of another for a maneuver. You save propellant mass by executing an identical-delta-v maneuver using a less massive craft, because there is less mass the inertia of which you need to overcome, and as a consequence have a larger delta-v budget with an identical propellant mass with the lighter craft (such as the LM vs the CSM). But to go from, say, a particular lunar orbit to the lunar surface will still require exactly the same change of velocity, which is what delta-v is; all that changes is how hard it is to attain that change of velocity. $\endgroup$ – user Feb 2 '17 at 15:54

I cannot see into their minds fifty years ago, but I'm sure that executing the rendezvous itself was a significant factor. It is far easier to achieve a rendezvous with a circular or nearly circular orbit. Six elements to match become four, and there are simple approaches to phasing and terminal rendezvous in a circular orbit.

  • $\begingroup$ On Apollo 11, the rendezvous was conducted with Columbia in a 56.6 x 62.5 n mi orbit; I don't know whether that's close enough to circular to discount the eccentricity. Those numbers seem to be typical of the Apollo missions. $\endgroup$ – Russell Borogove Feb 2 '17 at 0:21
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    $\begingroup$ Completely discount, no. Use circular orbit rendezvous techniques, yes. $\endgroup$ – Mark Adler Feb 2 '17 at 0:24

Having the CM in a low, circular orbit means it completes an orbit more often. I guess this gives the LM more frequent launch windows in case they need to abort?

  • $\begingroup$ You could pick an elliptical orbit with the same (or similar) orbital period as the near-circular orbits used. Even so, the difference would likely be on the order of tens of minutes at most; in other words, likely anything but a showstopper even from the emergency evacuation from the surface scenario. I suspect the critical decisions were the quick "stay for T1" go-around Mission Control (T1 being the point at which the CSM was out of range of the LM to re-rendezvous), followed by the more thorough "stay for T2" (CSM coming into range after its first orbit after LM touchdown). $\endgroup$ – user Feb 3 '17 at 15:13
  • $\begingroup$ Any elliptical orbit with the same perilune will have a longer period. So you're now talking about how low you want to go.. in the op case of 50k its going to be a much longer period irregardness of the perilune. $\endgroup$ – Innovine Feb 3 '17 at 16:01

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